Frequently, excessive voltage or current is applied across service lines that deliver power to residences and commercial and institutional facilities. Such excess voltage or current spikes (transient overvoltages and surge currents) may result from lightning strikes, for example. The above events may be of particular concern in telecommunications distribution centers, hospitals and other facilities where equipment damage caused by overvoltages and/or current surges and resulting down time may be very costly.
Typically, sensitive electronic equipment may be protected against transient overvoltages and surge currents using Surge Protective Devices (SPDs). Since SPDs may be relied upon to protect sensitive and potentially costly electronic equipment, manufacturers of such devices may perform stringent performance testing on SPDs using one or more testing regimens. For example, high current generators may be used to produce current waveforms in order to experimentally simulate lightning related surges.
Some testing standards have determined specific current curves for simulating a surge event, such as, for example, a lightning strike. An example international testing standard includes IEC 61643-11 “Additional duty test for test Class I” for SPDs (Clause 8.3.4.4) based on the impulse discharge current waveform defined in Clause 8.1.1 of IEC 61643-11, typically referred to as 10/350 microsecond (“μs”) current waveform (“10/350 μs current waveform”). For example, brief reference is made to FIG. 1, which is a plot illustrating the 10/350 μs waveform. The 10/350 μs current waveform may characterize a current wave in which the maximum current (100%) is reached at about 10 μs and the current is 50% of the maximum at about 350 μs. Under 10/350 μs current waveform, the transferred charge, Q, and specific energy, W/R, to SPDs should be related with peak current according to one or more standards. For example, the IEC 61643-11 parameters to Class I SPD test are illustrated in Table 1, which follows:
TABLE 1Parameters for Class I SPD TestIimp within 50 μsW/R within 5 ms(kA)Q within 5 ms (As)(kJ/Ω)2512.5156201010012.56.25391052552.56.2521110.50.25
However, the required Q and W/R can be achieved within a time interval several times lower than that corresponding to a zero impulse current flow.
Conventionally, high current may be produced by the discharge of capacitors through an RL circuit. In order to obtain unidirectional current, critical and/or overcritical damping of the RLC circuit may be used and may be achieved by using a damping resistance R≧2 (LC)0.5. For example, brief reference is made to FIG. 2, which is a graph that illustrates current waveforms. A critically damped RLC circuit may be represented by curves “a” and “b” illustrated therein. Higher peak currents for the same generator energy can be achieved by an undercritical damping circuit (R<2(LC)0.5). However, as illustrated by curve “c”, the current waveform may be oscillatory. In order to get an unidirectional current a crowbar device may be used, usually in conjunction with a controllable three electrode gap, ignited at the instant of the crest value of the current so as to achieve a non-oscillating waveform, such as shown in curve “d”. Such crowbar devices may be successful at producing current waveforms according to one or more testing standards. However, such devices may require calibration and synchronization for different types of varistor based SPDs. Because they may be configured to trigger at the peak current. As such a different generator may be used for each type of SPD.
Additionally, the application of greater Q and W/R than is specified in the test standard may result in excessive electrical and thermal stress on the SPD being tested. For example, the SPD being tested may experience excessive electrical and mechanical stress corresponding to the ‘tail’ of the 10/350 μs waveform. Such stress may be particularly critical regarding the performance of varistors.